The government owns rights in the present invention pursuant to grant number N01-C0-74101 with ABL.
1. Field of the Invention
The present invention relates generally to the fields of neurophysiology and neuropharmacology and more particularly to the action of neurofibromin-1 and its role in the survival of neural cells in the absence of neurotrophins.
2. Description of the Related Art
In humans, mutations in the neurofibromatosis 1 (NF1) gene lead to benign and malignant tumors of the peripheral nervous system and to abnormal distributions of melanocytes (Riccardi 1991; Gutmann and Collins, 1993). The common embryonic origin of these cell populations is the neural crest (Weston, 1970). The product of the NF1 gene, neurofibromin, is a 250 kDa protein that is widely expressed during embryogenesis (Daston and Ratner, 1992; Huynh et al., 1994). In the embryonic mouse dorsal root ganglia and CNS, increases in neurofibromin mRNA and protein coincide with periods of neuronal differentiation (Huynh et al., 1994). In the adult rodent, the brain and spinal cord are the predominant sites of neurofibromin expression (Daston et al., 1992).
Neurofibromin contains a 350-amino acid domain that has homology to mammalian (GAP) and yeast (IRA) GTPase-activating proteins that function as negative regulators of the p21ras oncoprotein (Ballester et al., 1990; Buchberg et al., 1990; Martin et al., 1990; Xu et al., 1990a). This GAP-related domain (GRD) of human neurofibromin can complement loss of IRA function and activate ras-GTPase in yeast (Ballester et al., 1990; Xu et al., 1990b). In addition, neurofibromin may regulate p21 ras activity independently of its GTPase-activating properties, as overexpression of neurofibromin can inhibit growth of cells transformed by the v-Ha-ras oncogene, which is resistant to GTPase stimulation (Johnson et al., 1994).
p21ras has been implicated in the neurotrophin response for embryonic vertebrate sensory and sympathetic neurons. During development, these neurons become dependent on specific neurotrophins for survival as they contact their targets and gain access to these molecules (reviewed by Korsching, 1993; Davies, 1994; Vogel, 1994). Nerve Growth Factor stimulates activation of p21 ras in embryonic chick dorsal root ganglion neurons (Ng and Shooter, 1993). Moreover, ectopic expression of activated p2lras protein can mimic the survival and neurite-promoting effects of neurotrophins in embryonic chick sensory neurons and in rat PC12 pheochromocytoma cells (D'Arcangelo and Halegoua, 1993). Conversely, in a more direct experiment, introduction of function-blocking ras antibodies abrogates the effects of neurotrophins on these neurons (Borasio et al., 1989; 1993).
NGF and its related neurotrophins: BDNF, NT-3, and NT4/5, bind cell surface tyrosine kinase receptors (TrkA, TrkB, and TrkC; reviewed by Snider, 1994). In rat pheochromocytoma (PC12) cells, NGF-mediated tyrosine phosphorylation of TrkA stimulates intracellular signalling pathways that are known to include activation of SHC, PLC-gammal, PI-3 kinase, and MAP kinases (Kaplan and Stephens, 1994). This body of results is consistent with the idea that the ras pathway is important, and may be essential, in neurotrophin/trk mediated signalling. Thus, trk receptors may directly activate proteins that regulate ras, including guanine nucleotide exchange factor and GAP (Li et al., 1992).
In addition, neurotrophins have been identified as affecting various neurodegenerative diseases and neurotrophin are administered therapeutically. For example, microencapsulated BDNF and other trophic factors have been suggested as a possible therapy of neurodegenerative disorders, in particular for peripheral neuropathies and CNS disorders where supplementation of neurotrophins retards or even prevents neural degeneration (Mittal et al., 1994). Neurotrophic factors have also been suggested for the prevention or delay of age related neurodegeneration or Alzheimer's disease (Rylett and Williams, 1994). Neurotrophic factors may also be used in the treatment of stroke, ischemia or other nervous system injury (Olson, 1993), amyotrophic lateral sclerosis (ALS) (Seeburger and Springer, 1993) as well as Parkinson's Disease, Huntington's Disease and spinal chord injuries.
However, the use of neurotrophins suffers from several drawbacks. For example, neurotrophins are unstable in vivo and high concentrations are needed in order to produce the desired effects. Unfortunately, high concentrations of neurotrophins may cause undesirable secondary effects. In addition, the mechanisms of sequestration of neurotrophic factors is unknown. Also the use of neurotrophins depends on a functional receptor, and because the receptor may be located at the axonal end of the neural cell, the receptor may be missing or damaged, especially in trauma or spinal chord injury. There still exists therefore a need for a method of therapy that prevents or delays neural cell degeneration due to neurodegenerative disease or other trauma.